Soft story configuration in structures is a type of construction where any one story of the building is more flexible (less stiff) when compared with other floors. This may be located at the bottom, or at any intermediate points, where the floor above or below it may be stiffer compared to itself. This accounts to be a weak element in the perspective of seismic forces.
Types of soft story buildings include apartments built over a parking garage or retail businesses with large windows, as well as multi-level commercial structures with floors that have large spaces, windows and doors. Soft story buildings most likely to collapse are those built on street corners or unstable soil. However, any multi-level residential or commercial building with unreinforced ground floor openings is a potential hazard during a quake, especially buildings constructed between the 1960s and 1980s.
Technically, a building is considered “soft story” if a floor is less than 70% as strong as the floor above it, or less than 80% as strong as the average strength (stiffness) of the three floors on top of it. This is because the weaker floor is more susceptible to lateral motion from the quake. The side-to-side earthquake motion can create enough stress to damage or collapse the floor. A single floor collapse can lead to the collapse of the entire building.
With an estimated 20,000 soft story buildings throughout Los Angeles, earthquake retrofitting is a serious matter. A new proposal could result in mandatory retrofitting requirements with a 30-year retrofit deadline. To date, only a relatively small number of property owners have taken action to improve the structural integrity of their buildings.
Many structural damages recorded due to earthquake had a major problem of change in stiffness and strength along their vertical configuration. It is not only essential to have symmetry along the horizontal direction, i.e. in the plan, but also in the vertical direction. This is a factor that assures lateral stiffness. Abrupt changes in the vertical plan should be avoided to the maximum.
The retrofitting process begins with an inspection by a qualified building contractor who has experience in soft story earthquake retrofitting. Several factors are taken into consideration when developing the retrofitting plan. Types of retrofitting include strengthening existing walls with braces or plywood, anchoring walls to the foundation and adding shear walls. The goal of the retrofitting is to make the building earthquake ready and safe by increasing the structure’s lateral stiffness and strength.
To get started today or for more information, contact us today to schedule a FREE on-site inspection by our team of soft-story experts or call us now at (818)287-8002.
Posted on Monday, June 3rd, 2019.